4-8-4. Unaddress Commands
The two unaddress commands are used by the
controller to simultaneously remove all talkers and
listeners from the bus. ATN is low when these
multiline commands are asserted.
UNL (Unlisten) - All listeners are removed from
the bus at once when the UNL commands is placed
on the bus.
UNT (Untalk) - The controller sends the UNT
command to clear the bus of any talkers.
4-8-5. Device-dependent Commands
The meaning of the device-dependent commands is
determined by instrument configuration. Generally,
these commands are sent as one or more ASCII
characters that tell the device to perform a specific
function. For example, M2 is sent to the Model 8020
to place the instrument in the external trigger mode.
The IEEE-488.2 bus treats device-dependent com-
mands as data in providing that ATN is high (false)
when the commands are transmitted.
4-8-6. Common Commands and Queries
Since most instruments and devices in an ATE
system use similar commands which perform identical
functions, the IEEE-488.2 document has specified a
common set of commands and queries which all
device must use. This avoids the problem in which
devices from various manufacturers used a different
set of commands to enable functions and report
status. The IEEE-488.2 treats the common commands
and queries as device dependent commands. For
example, *TRG is sent over the bus to trigger the
instrument. Some common commands and queries,
however, are optional; most of them are mandatory.
The following set of command groups ensure that
all devices communicate uniformly:
1. System Data - These commands are used to
store or retrieve information such as device identifi-
cation, descriptions and options. It is possible to
determine the manufacturer, model, and serial number
of the device under remote control.
2. Internal Operation - These commands include
such instrument operations as resetting, self-calibrat-
ing, and self-diagnostics of a GPIB device. The
device may respond to a calibration query to indicate
that the calibration was carried out successfully and
report any calibration errors that may have occurred.
The reset command sets the device-dependent func-
tions to a known state and must not affect the state
of the IEEE-488 interface, the Service Request Enable
register, or Standard Event Status Enable register.
3. Status and Event - These commands control
the status structure of the GPIB device and provide
a means to read and enable events. Included in
these commands are Clear, Event Status Enable,
Power-on Status, and Service Request Enable.
4. Synchronization - The operation of the devices
within the system are synchronized with these com-
mands. Included is a Wait to Continue command
which forces the devices to complete all previous
commands and queries. The Operation Complete
command tells the device to set bit 0 in the Standard
Event Status register when it completes all pending
operations.
5. Device Trigger - These commands enable a
device to be triggered and specify how it responds
to the trigger message. The Define Device Trigger
command stores a sequence of commands which
the device will follow when the Group Execute Trigger
(GET) is received.
6. Stored Settings - These commands are used
to save the state of the device under control, to be
used at a later time. The Save command stores the
present state of the device in the device’s memory.
If there is more than one location in which this data
can be stored, the command is followed by a number
which designates the storage register to use. The
Recall command restores the state of the device,
as stored in its memory from the previous Save
command. As with the Save command, the Recall
command must be followed by a number to specify
the register from which the stored settings are to
be recalled.
4-9. DEVICE LISTENING FORMATS
This paragraph discusses the formatting of <Program
Message> elements received by a device from its
system interface. Allowable IEEE-488.2 <Program
Message> is composed of sequence of <Program
Message> units, each unit representing a program
command. Each program command is composed of
a sequence of functional syntactic elements. Legal
IEEE-488.2 program commands are created from
functional elements sequences.
Some commands of universal instrument system
application have been defined by the IEEE-488.2.
They are the common commands; these commands
and queries are specific path selections through the
functional syntax diagram as specified in the IEEE-
488.2 standard. The remaining commands are de-
vice-specific and are generated by the device designer
Model 8020 IEEE-488.2 Operation
Page 4-9
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